1. Field of the Invention
The present invention relates to a method of preparing a transparent polymer material comprising a thermoplastic polycarbonate and nanoparticles having a form factor strictly greater than 1.0.
The invention applies typically, but not exclusively, to the fields of optics, in particular optical articles of the optical lens type for viewing instrumentation, a helmet visor, or an ophthalmic lens, and to optical articles of the automobile glass type.
More particularly, the term “ophthalmic lens” is used to designate lenses suitable in particular for fitting to an eyeglass frame and having the function of protecting the eye and/or correcting vision, such lenses being selected from afocal lenses, single-vision lenses, bifocal lenses, trifocal lenses, and progressive lenses.
The term “automobile glass” is used to designate not only transparent external bodywork elements such, for example, rear windows, quarter windows, side windows, glazed roofs, glass for headlights or indicator lights, but also transparent elements for inside the cabin, such as for example glass for the dashboard, for dials, or for screens.
2. Description of Related Art
Polycarbonate presents advantages that make it particularly advantageous for optics, in particular excellent transparency, excellent resistance to impacts, high refractive index, and great lightness. In contrast, as its main weak points, it is not very rigid and it is very sensitive to scratching and abrasion. In addition, the linear coefficient of thermal expansion (LCTE) of polycarbonate is relatively high, thereby running the risk of high levels of deformation in said polymer under the action of temperature.
In order to improve the mechanical properties of a polymer, in particular its rigidity and its ability to withstand abrasion and scratching, it is known to add mineral nanoparticles to the polymer, in particular mineral nanoparticles having a form factor that is strictly greater than 1.0.
An example of a method of incorporating such mineral nanoparticles is described in document. EP 1 632 536. Said nanoparticles are incorporated in a thermoplastic polymer matrix of the polycarbonate (PC) or polymethyl methacrylate (PMMA) type while in the molten state. In addition, it is also known that such nanoparticle incorporation may be performed in a mixture of polymers.
Nevertheless, when incorporating mineral nanoparticles in a thermoplastic polymer material or a mixture of polymers, the nanometric size of the mineral particles gives rise to a phenomenon of said nanoparticles aggregating. As a result, the resulting polymer material presents a loss of transparency, thereby making it difficult, if not impossible, to use in the fields of optics. In addition, incorporating mineral nanoparticles may give rise to a chemical degradation of the polymer material and thus of its mechanical properties, such as for example its ability to withstand impacts. It is also possible for coloring to appear, in particular yellowing.